1. Field of the Invention
This invention relates to a novel process for preparing polydiene polyurethanes, especially polydiene polyurethanes having significantly improved tear strength.
It is well known that polydiene polyurethanes comprise polydiene glycol, diisocyanate and chain-extending agent.
2. Description of the Prior Art
Polybutadiene polyurethanes, for example, which are made from polybutadiene glycols prepared by radical polymerization of butadiene with use of an initiator such as hydrogen peroxide have been generally used and well known. However, as they have poor mechanical properties, various methods for improving them have been proposed. For example, the method of incorporating reinforcing fillers such as carbon black, silica and zinc oxide, etc., to the raw polybutadiene polyurethane compositions, the method of increasing the amount of hard segments by isocyanate components or chain-extending agents, the method of blending polyether glycol, and the method of complicating networks by epoxide, sulfur and peroxide and others have been proposed. However, these proposed methods have little effect on the improvement of low tear strength which is the main disadvantage of polybutadiene polyurethanes, and furthermore affect adversely their good rubber like, elasticities at low temperatures and their water-durability. On the other hand, the polydiene glycol prepared with the use of lithium-based catalyst has been seldom used, because the undesirable abnormal gelation which takes place sometimes upon the preparation of prepolymer will cause damage to these mechanical properties.